The present invention relates an apparatus and method for measuring fluid flow, and more particularly, to determining blood flow rates in hemodialysis arterio-venous (A-V) shunts by dilution techniques, including thermodilution.
Hemodialysis is a process by which an artificial kidney replaces the function of a kidney in a patient. Blood is removed from the patient through a patient access such as an arterio-venous (A-V) shunt, passed through a dialyzer and returned to the patient access for normal circulation through the vascular system of the patient.
Arterio-venous shunts used in hemodialysis are surgically created by insertion of a specially designed tube (artificial graft or autologous vein) between a peripheral artery and a vein, or by connecting a peripheral artery to a vein to create a native (shunt) fistula. The A-V shunts are designed to supply blood for the dialysis process. Typically, hemodialysis needles are inserted into the A-V shunt during a hemodialysis session and the shunt must provide enough blood flow to allow the dialyzer to effectively perform blood purification.
Because the A-V shunt is seen as a foreign object by the body, in many cases the hemodynamic conditions increase the probability of stenosis development. The stenosis may thrombose if timely intervention such as angioplasty or surgery is not performed. During an angioplasty procedure, the radiologist tries to restore the flow by a procedure such as balloon angioplasty. An estimation of intervention success is usually based on an X-ray visual picture of the narrowing, rather than measurement of the flow that presumably has been restored. More than 20-30% of these costly interventions are not successful. That is, the patient access thromboses or the angioplasty did not change the flow rate or flow rate drops back to the initial value within one month.
Another problem exists when the blood flow in an A-V shunt (mostly in native fistulae) increases to a very high value such as 3 to 4 l/min (liters per minute). This high flow rate can be dangerous because of possible heart overload. In this situation the surgeon should intervene to decrease the blood flow. Such change must be well-controlled: if the flow rate is decreased too much, there is a risk of thrombosis in the shunt if the flow rate decrease is too small, the danger of heart overload remains.
So the need exists to determine the blood flow rate in A-V shunts during angioplasty, surgical and other interventions for immediate assessment of intervention quality.
Commonly used methods to measure blood flow rate in biomedical diagnostic and research applications include transit-time ultrasound, Doppler ultrasound, electromagnetic, nuclear magnetic resonance and x-ray fluoroscopy principles. However, it is difficult to use these procedures routinely during angioplasty procedure on an A-V shunt.
A well-accepted blood flow measurement technique employing indwelling catheters is the indicator dilution method, often named Stewart-Hamilton method, after the inventors who pioneered this family of methods in the late 19th and early 20th century. In this method, an additional element is introduced into or extracted from the blood stream, or a blood property is changed (the xe2x80x9cindicatorxe2x80x9d). A calibrated sensor placed downstream from the point of indicator introduction measures the absolute concentration of the indicator. Via well-known equations, one can derive the flow rate. These methods are widely used for cardiac output measurement using pulmonary artery catheters.
Besides their use for cardiac output, the use of thermodilution methods is also known to measure blood flow in arteries [Ganz 64] and veins [Ganz 71]. An arterial system is characterized by high downstream flow resistance due to arterioles and capillaries. A venous system is characterized by high upstream flow resistance due to venulas and capillaries.
In contrast to arteries and veins, the flow resistance of A-V hemodialysis shunts is not concentrated in known upstream or downstream locations. Instead, the location of the flow resistance depends upon the condition of the A-V shunt. As a consequence, the injected indicator can alter flow in the shunt in an unknown manner. Additionally, the dynamic range of blood flow that should be measured in the A-V shunt can vary approximately 100 fold (50 ml/min-5000 ml/min). Thus, the measurement of flow in A-V shunts involves special requirements that do not exists in natural arterial and venous systems.
Therefore, a need exists for determining the blood flow rate in an A-V shunt having a relatively large dynamic range of blood flow rates during the angioplasty, surgical and other interventions, for immediate assessment of intervention quality. The need also exists for a method and apparatus for determining the blood flow rate in an A-V shunt without requiring extensive retraining of personnel.
The present invention provides a method and apparatus for measuring the blood flow rate within an A-V shunt, wherein indicator dilution techniques are employed. The present invention accommodates the unknown flow resistance within the A-V shunt by adjusting the measured flow rate to provide a flow rate within a predetermined margin of error.
The invention also provides for an improved measurement accuracy of A-V shunt flow rate by an adjustment based on measurement of the injected indicator flow.
The invention further provides for improved measurement accuracy of an A-V shunt flow rate by a system configuration, wherein the system determines, or measures, the flow rate of the injected indicator and rejects or accepts the measured flow rate in the A-V shunt based on the flow rate of the injected indicator.
The invention further discloses improving the measurement accuracy of A-V shunt flow rate by the introduction of the indicator at different flow rates.
The invention also contemplates the use of a flow restrictor in the indicator flow path, the flow restrictor selected to limit the rate of flow of the injected indicator. That is, the flow rate of the injected indicator is limited by the structure of the indicator flow path.